In electronic time division type switching systems, such as PBXs, the system line or port circuits are mounted on printed wiring boards which, in turn, are inserted into and electrically connected to equipment called a port carrier. The port carriers have slots or recesses into which the boards may be inserted; and contacts at the back of the carrier electrically interconnect backplane wiring of the carrier with cooperating board contacts. With current semiconductor technology, each port circuit is relatively small in size and a plurality of port circuits, typically four (4) or eight (8), are mounted on a single board. Each port circuit is assigned to a different system time slot; and each port circuit serves a call by exchanging signals, during the occurrence of its assigned time slot, with the system's switching network over a network bus as well as with the system's processor or controller via an I/O bus. Each port circuit serves a station device, such as a telephone, over an associated transmission path, such as a tip and ring pair.
A recently proposed system for providing integrated voice and data service, has both a telephone and a data device, such as a terminal, at each station. This is shown in the publication "Frame-Mode Customer Access to Local Integrated Voice and Data Digital Networks," authored by Accarino et al, from the proceedings of the 1979 International Conference on Communications, pages 38.5/1-7. FIGS. 2 and 3 of Accarino et al disclose how a telephone and a terminal at a station can be served over a single transmission path by the use of signals encoded in the frame format shown in FIG. 3 of Accarino et al. Each frame includes two PCM (Pulse Code Modulation) sample fields. One of these fields is associated with a telephone, the other serves a data terminal. The transmission path from each such station terminates at a single port circuit (LTE) of the switching system.
An advantageous way for a single port circuit of a PCM switching system to serve calls from two station devices over a single transmission path using the frame format of FIG. 3 of Accarino et al is to assign two system time slots to each such port circuit. Each of the two assigned PCM system time slots is associated with a different one of the two PCM sample fields of FIG. 3 of Accarino et al. With this arrangement, the "speech" signals from the telephone are received in a first one of the PCM sample fields. The required call connection through the PBX for the speech signals is served during a first one of the two system time slots assigned to the port circuit. The signals generated by the other station services such as a data terminal are received by the port circuit in the other PCM sample field. The required call connection for the data terminal is served during the other time slot assigned to the same port circuit.
PCM type time division switching systems are normally designed so that a fixed number of time slots are applied to a port carrier even though the carrier is capable of accommodating different types of port circuits. A system equipped for receiving station information in a frame format similar to that of FIG. 3 of Accarino and for providing integrated voice-data service may have, for example, 64 port circuits on each carrier. These 64 port circuits may be mounted four per board so that each carrier has 16 port boards. Each of the 64 port circuits on a carrier is assigned to two different system time slots and thus, each carrier receives the 128 PCM time slots and 128 I/O singals required to serve its 64 port circuits so that they can provide a first type of service, such as voice-data service.
A system as above characterized can also provide second type of service, such as voice only service, or data only service by using less complex port circuits mounted so that there are 8 port circuits per board or 128 port circuits per carrier. Each such port circuit requires only a single time slot for the second type of service hereinafter termed voice only service. The system operates in the same manner as above described, insofar as system time slots and carriers are concerned, since each carrier still receives and distributes 128 time slots and I/O signals to the port circuits requiring them.
It is desirable in the operation of time division switching systems that time slots and the I/O buses be used as efficiently as possible in spite of varying customer service demands. In a system equipped for voice only service, time slots and I/O buses are normally efficiently used since a new port carrier would not be added to serve additional stations while there are unused time slots and port circuits on the existing carriers. In other words, even though service demands of the customer may vary for voice only service, these varying demands may be accommodated by adding or removing port circuit boards to the existing carriers as required. A new port carrier would be added only if all port circuits on the port boards of the existing carriers are assigned to existing stations.
The problem of efficient time slot and I/O bus utilization is more complex for systems that receive information from a pair of station devices over a single line in a frame format similar to FIG. 3 of Accarino and provide voice-data service to extend a call connection from the port circuit serving such a line. The system time slots are used efficiently only if each station has both a telephone and a data terminal so that the two time slots assigned to each port circuit are used. A problem arises under circumstances in which service requirements vary to the extent that, at certain times, a large number of stations may have only a single station device, such as only a telephone. Under these conditions all time slots are not efficiently utilized since each port circuit of the voice-data type still has two time slots assigned to it. The problem is aggravated, and can impair efficient service, under conditions in which many such stations are equipped with only a telephone or only a data terminal. This can lead to a situation in which extra carriers will have to be added even though there are unused time slots in existing carriers. In extreme cases, it may be required to add another module and a time multiplex switch to provide acceptable service, even though time slots exist and remain unused in port circuits of the first module.
It is therefore a problem to use time slots efficiently as the customer demands vary in time division switching systems having the capability of providing integrated voice-data service where each port circuit is assigned to two time slots.